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EFFECT OF HYDROTREATING ON THE STABILITY OF SYNTH ETICCRUDE FROM WESTERN CANADA

P. Rahimi and C. Fairbridge, Nation al Centre for Upgrading Tech nology ,

1 Oil Patch Drive, Suite A-202, Devon, Alberta, CAN ADA , T9G lA 8

M. Oballa, C. Wo ng and A. Somogyvari, NOV A Research and Techn ology

Corporation.2928-16th Street N.E., Calgary, Alberta, CA NA DA . T 2E 7K 7

Keywords hydrocracking, distillates, hydrotreating,colour storage stability

ABSTRACT

The storage stability of distillates from the hydrocracking of Westem Canadian bitumenatmospheric residue was studied over 60 days. naphtha-jet and gas oil fractions of the

hydrocracked distillates were show n to be un stable with respect to the formation of existent gum ,

total insoluble materials and asphaltene. The storage stability was significantly improved when

these two fractions were mildly hydrotreated. The data were used to generate correlations that

predict the stability of synthetic crudes.

INTRODUCTION

The refining industry in North America is consistently moving towards the utilization of heavier

feedstocks for the production of synthetic crud e. Current production of synthetic crude oil fromoil sands and heavy oils is achieved by hydrogen addition as well as carbon rejection

technologies. The primary products from these heavy feedstocks require different degrees of

hydrotreating to obtain transportation fu els that m eet current specifications. T he hydro processingof bituminous materials and residues results in coke deposition on the catalyst as well as sludge

formation in the product oil [11. Th e products obtained are rather unstable, generally described as

storage instability. Storage stability of hydrocarbon fuels therefore refers to their tendency to

produce coloured species, soluble gums, and insoluble sediment during storage [2]. The storage

and thermal stability of liquid stocks especially diesel and jet fuel, have been topics of intensestudy especially by the military [3, 41. To our knowledge, there has been very little study on the

stability of hydrocracked products from the residues of C anadian heavy o ils or the synthetic crudeso blended after hydrotreating the hydrocracked distillate fractions.

The objective of the study was to quantify the rate of deterioration of the hydrocracked material

and the hydrotreated distillates in terms of colour change and formation of soluble gums and

insoluble materials, as well as to establish the presence or absence of the precursors to instability

in the liquid fractions.

EXPERIMENTAL

The feedstock for this study w as 50 /50 Cold Lake Llyo dmin ster atmospheric residue (399'C +).

The hydrocracking of the residue to distillate fractions was performed on a commercialNiMo/AlzOs catalyst using a continuous bench scale stirred tank reactor. The experiments were

carried out using the following conditions: Pressure < 20,685 kPa, temperature < 450'C and

LHSV 1 h-'. Th e catalyst was presulphided and conditioned for 1 20 hours. T o obtain therequired amoun t of distillate products the experiment was continued for another 16 hours. Th eproduct samples had to be analyzed as soon as they were generated and distilled to give the

fractions required for further processing (hydrotreating) or analysis. Distillation of thehydrocracked materials was carried out in order to obtain fractions which were tested for storage

and colour stability and further characterized to identify materials causing instability. The

hydrocracked materials were distilled into naphtha, jet-fuel, diesel, gas oil and Residue. The

naphtha and jet-fuel fractions were analyzed for both colour and storage stability and then

combined as feedstock for hydrotreating. Similarly, diesel an d gas oil fractions were analyzed for

colour and storage stability and then com bined as feedstock for hydrotreating. The catalysts usedfor naphtha-jet and diesel-gas oil hydromating experiments were C-411 and C-424 catalysts

respectively. Detailed experimental procedures and equipment are described elsewhere [5].

Analytical M ethods and Analyses of Samples

Specific gravities were determined in triplicate at 15.5 C on a PaarDM A 4 8 instrument. Dynamicviscosities were determined in triplicate at 40°C using a Brookfield DV JI instrument. Heptane

insoluble asphaltenes were determined using the method of Pearson et.al. [6]. Existent gum was

determined by the jet evaporation technique according to AST M D 381-86. Hydrocarb on-types

were determined in low boiling mP-2 49 'C) distillates by the fluorescent indicator adsorption

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method according to ASTM D 1319-77 and gave the volume percent of saturates, olefins andaromatics. Th e aniline point of the various distillate fractions was determined according to

ASTM D 61 1-82. Bromine N umb ers were determined according to procedures in A STM D 1159-

82. Carbon, hydrogen and nitrogen were determined using a CHN Analyzer. Trace nitrogen was

determined on an Antek 771 pyroanalyzer coupled to an Antek chemiluminiscent nitrogendetector. Basic nitrogen was determined according to procedures outlined in UOP Method 269-70T. Sulfur was determined on a Leco SC-132 sulfur analyzer. The colour of the petroleum

products was determined by the Lovibond Tintometer method as described in IP 17/52. Theoxidation stability of middle distillate fractions was determined according to ASTM D 2274-88

and these procedures have been found useful for estimating the storage stability of distillate fuelsboiling between 1 75 and 370'C. Th e procedures in ASTM D 4625-87 were followed for the

prediction of storage stability. Hydrocarbon-type analyses on the liquid samples were performedusing a modified ASTM D 2007-75 and ASTM D 2579-78 procedures. The saturates and the

aromatic fractions were analyzed by low resolution mass spectrometry by modifications to

procedures in ASTM 3239 and ASTM 2786-71. Th e polars fraction, which contained mostly

nitrogen heterocycles, was similarly analyz ed via high resolution mass spectrometry.

RESULTS AND DISCUSSION

Effect of Hydrocracking:

The hydrocracking of the feedstock resulted in a 66.7% conversion of material boiling above

524'C (Table 1). Th e asphaltene content which is mo re or less a measure of the hydrogendeficiency of an oil was reduced to one third of its original value after hdrocracking and mainly

remained in the 524 C+ fraction. The increase in the hydrogen to carbon ratio after hydrocracking

was synonymous with the decrease in the asphaltene content. The viscosity and density of thehydrocracked materials decreased as expected. Of particular interest is the ratio of non-basic

nitrogen to basic nitrogen in the hydrocracked sample. Some nitrogen compounds have been

known to contribute t fuel instability, while others are inert. As show n in Table 1 hydrocracking

reduced the total nitrogen in the feed, but the basic nitrogen increased.

Effect of Hydrotreating

Th e hydrotreating was can ied out on the naphtha-jet fuel fraction as w ell as on the diesel-gas oilfraction. For simplicity, these hydrotreated fractions are referred to as hydrotreated naphtha-jetand hydrotreated Gas Oil respectively. The hydrotreating step not only removed the heteroatoms

S , N O ,ut also refined and stabilized the products. Sulfur was released in the form of HzS,

nitrogen in the form of NH3, and oxygen in the form of H2 0. Th e following tests were performed

to determine the effect of hydrotreating on the stability of different distillate fractions.

StabilityTests:

Total Insolubles: The oxidation stability results performed on naphtha-jet fraction re shown in

Table1.

The data indicate that the total insolubles were much higher in the unhydrotreatedmaterial than in the hydrotreated samples. This data confirmed that hydrotreating stabilized the

reactive hydrocarbons thereby rendering them less reactive than they otherwise would have been.

The effects of hydrotreating on the long term storage stability of different fractions re shown in

Table 2 and Table 3. In the unhyd rotreated naphtha jet fraction, after 60 days of storage time, the

total insolubles increased from 0.03 to 19.2 mg/lWml whereas in the hydrotreated sample, the

total insolubles were at 0.25 mg/100 ml after storage for 6 0 days. In the unhydro treated gas oilfraction, the total insolubles were 15.9 mg/100 ml after 60 days of storage, while for the

hydrotreated sam ple, the total insolubles were only at 3.6 m g/100 ml after 60 days of storage.

Existent Gum: It is generally believed that the products of initial oxidation, probably peroxides,

catalyze the oxidation of normally less reactive hydrocarbons to increase the rate of gum

formation. Mos t of the oxidation products a re said to be soluble in naphtha but decom pose duringevaporation to give gum that is largely composed of acidic material. Some of the studies show acorrelation between gum content and total nitrogen content, while in other studies the gum

content is said to increase with boiling range. The results in Tables 2 show that the existent gum

in the naphtha-jet fraction decreased from 8.8 mg/100 ml before hydrotreating to zero after

hydrotreating. After storage of the unhydrotreated fraction for 60 days, the existent gum was 142mg/100 ml as opposed to a hydrotreated sample stored for the same 60 days which had 0.80

mg/lOO ml.Undou btedly, hydrotreating affect s the existent gum content of naphtha -jet fractions.

Colour : The results of the colour test are shown in Table 1. Colour is one of the mostdistinguishing characteristics of untreated hydrocracked distillates. On distillation after

hydrocracking, the naphtha-jet fraction is from clear to pale yellow. How ever, when exposed to

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Ordinary rw m conditions, this hydrocracked fraction will start to darken and also to dep osit gum.Colour change is therefore indicative of the aging of the sample. Th e naphtha-jet fraction before

hydrotreating had a colour number of 0.5 to 1.0 and after hydrotreating the colour number wasreduced to zero The gas oil fraction had a colour num ber of 3.5 to 4.5 before hydrotreating and

after hydrotreating the colour n umber was reduced to 1.5 to 2.

viscosity he viscosity of the fractions (Table 2 and 3 is influenced m ore by storage time than

by hydrotreating. Both the naphtha-jet and the gas oil fractions showed an increase in viscositywith storage time.

Aniline Point There is an inverse relationship between aniline point and aromatic content.

Aniline Point is usually increased slightly with the molecular weight and boiling point of asample, and rapidly with the paraffinicity of oil samples. Th e higher aniline point value denotes alower aromatic content and a higher paraffin content. Tables 2 and 3 show a higher anilinecontent after hydrotreating for both the naphtha -jet and the gas oil fractions. In Table 4, the

analyses for paraffins and aromatics confirm the above conclusions. Overall, themonocycloparaffins are being converted to dicycloparaffins, leaving a decrease in the paraffinic

content after aging. Both the mono- and di-aromatic constituents of the distillates increased at the

expense of the paraffinic components, which suggests that saturated paraffinic components

undergo some form of condensation with aromatic constituents.

SUMMARY

The work presented here was undertaken to address the issue of the storage stability of middledistillates obtained from Western Canadian heavy oilhiturnen. Fuel stability is the generalresistance of a fuel to change. Two types of stability were investigated. The first was storagestability, which reckons the ability of the fuel to stay in storage for a long period of time with

little deteriora tion. Th e second was thermal stability wh ich is the ability of the fuel to resist

with little deterioration high temperature stress for a short period. The study results show that

the deterioration o f the fuel through storage manifested itself in colour change , develop me nt ofsoluble and insoluble gum, and change s in the physical and c hemical properties of the fuel, like

viscosity, density, nitrogen, sulfur, aromatics and asphaltene content. The results obtained alsosuggest that hydrocracked or thermally cracked materials should be hydrotreatedproc essed asquickly as possible. The detail hydrocarbon-type analyses of differe nt fractions before and after

hydrotreating showed that hydrotreating reduced refractory materials, including polar

com poun ds, that caus e storage instability.

ACKNOWLEDGEMENT

Our thanks go to the management of NCUT and NOVA Research Technology

Corporation fo r financing the-project and publication of the paper.

REFERENCES

1 M. F. ymoniak and A. C. Frost, Oil & G a s Journal, Mar. 17.1971.76.2- J. Ritchie, J. Inst. Petr., 1965,Vol. 51, 296.3- 0 K. Bahn, D.W. Brinkm an, J.B. Green and B. Carley, Fuel, 1987, Vol. 66, 1200.

4 J. V. Cwney , E. J. Beal, M.A. W echter, G. . Mushrush and R. N. Hazlett, Preprints, Petroleum

Chemistry D ivision, 1984, Vol. 29, 1003.

5- M.C. Oballa, C. W ong and A. Somogy vari, ‘%olour and Storage Stability of Synthetic Crudes

Distillates”, Final Report, for Energy Mines and Resources Canada, 1992, SCC File N o :

6 .D. Pearson, G.S. Huff and S.G. Garfeh, Anal-Chem., 1986, Vol. 58,32650634.23440- 1 9014

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TABLE 1: COLOUR AND STORAGE STABILITY OF SAMPLES

(1) Feedstock

2)Hydrocracked product

,

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TABLE 2 NAPHTHA-JET FUEL FRACTION EFFECT OF HYDROTREATING

ON STORAGE STABILITY

PROPERTIES

IBP 249 CUnhydrotreated IBP 249OC HydrotreatedAtTimeO Afteradays AtTimeO I Afteradays

TABLE 3 DIESEL GAS OIL FRACTION EFFECT OF HYDROTREATING

ON STO RAGE STABILITY

PROPERTIES

249 524°C Unhydrotreated I 249 524'C HydrotreatedAtTimeO After60days I AtTimeO Afteradays

TABLE 4 MASS SPECTROMETRY ANALYSIS RESU LTS OF THEHYDROCRACKED SAMPLES

Before

2 97 3 35 0 97

3 89 5 08 0 38

HT

Storage

19 02

47 45

25 80

Hydrotreated